Add support for referencing environment variables in config

[plombardi89]

Docker (and its cousins) do cool stuff like add link information to a containers environment variables, for example, if you specify a --link mysql:db' in therun` command you will be given that information in the container such that it can be retrieved via an environment variable like $DB_PORT_3306_TCP_ADDR.

This feature would expose a configuration file syntax similar to Ruby On Rails <%= ENV[''] => configuration string that will automatically query the environment variables if present rather than rely on a hard coded value.